In article ,
C. J. Clegg wrote:

So, my questions are...

Take my answers as best-guesstimates, please, rather than as gospel!

1. How do I determine the overall antenna length that will give me the
most efficient (which is to say, the least inefficient) performance across
the 4 to 9 MHz range?

A range greater than 2:1 means that you're almost certainly going to
run into at least one operating frequency at which the antenna itself
is an extremely difficult load (very low, very high, and/or very
reactive) and that most of your power is going to end up in the
terminating resistor.

One way to evaluate different lengths would be to model out such an
antenna using NEC2/NEC4 or the like. Vary the antenna-length-to-
operating-frequency-wavelength ratio, and evaluate the amount of power
radiated vs. the amount of power dissipated in the termination
resistance at each. Then, given the specific frequencies at which you
actually want to spend most of your time operating, figure out which
length gives you the best overall efficiency for your own operating
conditions.

At a guess - and it's just a guess - I think you might get reasonably
satisfactory results by choosing a length which would give you a
folded dipole that's resonant right in the middle of your 4-to-9
range, or perhaps a bit longer than that. You'd avoid the "too short
to load up efficiently" problem at 4 MHz, as well as the "a full
wavelength long, and thus presenting a high feedpoint Z" problem at 9
MHz.

2. How do I determine the minimum power rating for the terminating
resistor for an antenna that will be driven by 100 watts maximum?

Ummm... I'd say that you'd need a resistor capable of dissipating 100
watts, continuous, when used under ambient-free-air conditions in the
highest operating temperature you'll encounter. I'd probably de-rate
it by at least 50% (200 watt resistor) just to be sure, especially if
you're going to be operating RTTY or any other high-duty-cycle mode.

One of the unfortunate things about a T2FD is that there are going to
be frequencies where most of your power warms up the feet of the birds
perching on the termination resistor :-(

3. Where can I buy a few noninductive terminating resistors that meet the
power rating determined in (2)?

You could probably stick a bunch of Caddock MP9100 power film
resistors, wired in series, on a chunk of aluminum heatsink and make
it work OK. Caddock sells direct, I believe, and if I recall
correctly Mouser carries many of their parts.

You might ask B&W if they'd be willing to sell a "replacement" for
their termination resistor assembly.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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